mongodb的aggregate

目的是找到满足一定条件的house被不通用户关注的用户列表

house-user:

{ 
    "_id" : ObjectId("5b17a412882b7f075496b2b6"), 
    "house_id" : 1.0, 
    "user_id" : 2.0
}
{ 
    "_id" : ObjectId("5b17a424882b7f075496b2b7"), 
    "house_id" : 2.0, 
    "user_id" : 2.0
}
{ 
    "_id" : ObjectId("5b17a42f882b7f075496b2b8"), 
    "house_id" : 2.0, 
    "user_id" : 5.0
}

house:

{ 
    "_id" : ObjectId("5b17a2e1c82389528e424f4e"), 
    "house_id" : 1.0, 
    "house_name" : "house1", 
    "city_id" : 1.0
}
{ 
    "_id" : ObjectId("5b17a309882b7f075496b2ae"), 
    "house_id" : 2.0, 
    "house_name" : "house2", 
    "city_id" : 1.0
}
{ 
    "_id" : ObjectId("5b17a32b882b7f075496b2af"), 
    "house_id" : 3.0, 
    "house_name" : "house3", 
    "city_id" : 1.0
}

aggregation script:

// Requires official MongoShell 3.6+
use mydb;
db.getCollection("test_house").aggregate(
    [
        { 
            "$match" : {
                "city_id" : 1.0
            }
        }, 
        { 
            "$lookup" : {
                "from" : "house_user", 
                "localField" : "house_id", 
                "foreignField" : "house_id", 
                "as" : "userInfo"
            }
        }, 
        { 
            "$unwind" : {
                "path" : "$userInfo"
            }
        }, 
        { 
            "$group" : {
                "_id" : "$userInfo.user_id"
            }
        }
      {            // 例子而已
         "$project":{
            "_id":0,
            "user_id":1
         }
      }
    ], 
    { 
        "allowDiskUse" : false
    }
);

参考链接：https://www.cnblogs.com/duhuo/p/6068879.html